1. Field
This invention relates to syringes and in particular to plastic syringes, especially those intended for dispensing viscous medicament compositions.
2. Prior Art
There are problems associated with the manufacture of prefilled syringes in which satisfactory solutions have not been found. Normally, the discharge end of the syringe is first capped, the medicament is placed in the barrel to an appropriate height and the plunger is then introduced until the plunger tip with its seal comes into contact with the medicament. The air in the barrel above the medicament must be expelled in some manner during this final step. This can be accomplished either by providing an air vent in the plunger tip seal or by manually inserting a wire or similar device between the barrel wall and the seal to generate an air vent while inserting the plunger, then withdrawing the wire. The former method is unsatisfactory since the medicament can escape through the air vent. The latter method is time consuming and would not lend itself to automated prefilling techniques. The latter method also would require that the seal on the plunger tip have a thin, flexible wiper configuration but such a seal is readily subject to deformation particularly when made of plastic, which would result in an unsatisfactory seal. Seals made of rubber often are undesireable since the medicament or its vehicle may react in an unfavorable manner.
There have been a number of syringes which have been designed for metered dosage of a medicament such as those typified in U.S. Pat. Nos. 2,764,981, 2,856,925, 2,869,541, 2,875,761 and 3,934,586, all of which show various stop means on the plunger for permitting metered doses to be administered. U.S. Pat. Nos. 3,563,240 and 4,153,056 represent syringes in which fine adjustment of metered dosages are achieved by the use of a ring threadedly attached to the shaft of the plunger.